Conventionally, there have been devices that compress digital content and transmit the compressed digital content. For example, conventional devices transmit digital content compressed in an electronic publication (EPUB) format to a user terminal such as a personal computer (PC) or a cellular telephone. Examples of the digital content include video, music, books, and dictionaries. The compressed digital content is extended in the user terminal. Then the extended digital content is reproduced in the user terminal. In the user terminal, when a user wishes to view an item including a predetermined retrieval character string among items included in the digital content, the retrieval character string is searched in the digital content.    Patent Literature 1: Japanese Laid-open Patent Publication No. 11-110187    Patent Literature 2: Japanese Laid-open Patent Publication No. 2000-216685    Patent Literature 3: International Publication Pamphlet No. WO 2006/123448